skins2-howto.xml: Added two internal links

doc/skins/skins2-howto.xml

 <?xml version="1.0" encoding="iso8859-1" ?>
 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1//EN" "/usr/share/sgml/docbook/dtd/xml/4.1.2/docbookx.dtd">
 
+<!-- vim: set ts=2 sts=2 sw=2 et -->
+
 <article id="skin-creation-howto">
 
 <articleinfo>
@@ -84,7 +86,7 @@ difficulty to understand how VLC skins work.</para>
 <sect1>
 <title>The bitmaps</title>
 
-<para>Basically, you need one bitmap file (in PNG format) by state of control. For example, with a Image control you need 1 image, with a Button control you need 3 images (for the up, down and mouseover states). Of course, the same bitmap file can be used for many controls, provided you want to display the same image!</para>
+<para>Basically, you need one bitmap file (in PNG format) by state of control. For example, with a <link linkend="Image">Image</link> control you need 1 image, with a <link linkend="Button">Button</link> control you need 3 images (for the up, down and mouseover states). Of course, the same bitmap file can be used for many controls, provided you want to display the same image!</para>
 
 <para>The PNG format allows setting a transparency mask, which will be used wherever the image needs to be displayed (only non-transparent parts will be drawn). In addition, you can also specify a transparency color in the XML file: the bitmap will be considered as transparent wherever this color appears in the bitmap file.</para>